The invention is directed to a method for redundancy saving, error-correcting coding in digital radio link systems having multi-level modulation on the basis of forward error correction and is also directed to a following decoding.
M-earner quadrature amplitude modulation (M-QAM) is generally utilized in digital radio link systems to meet the demand for an efficient utilization of the spectrum. The trend of development is thereby to larger and larger numbers of levels, or steps, for example M=64 or 256. Systems having such a high number of levels, however, are significantly more sensitive to disturbing influences of all types such as, for example, noise, adjacent-channel interference and signal distortions than are systems having a low number of levels. The susceptibility of digital radio link systems to disturbance, however, can generally be reduced by an error-correcting coding. Only methods having forward error correction (FEC) are considered since the data rate is rigidly prescribed. For reasons of the greatly limited bandwidth, an increase in the transmission rate for the acceptance of the redundancy required for FEC is only justifiable up to about 3%. This great limitation requires the utilization of FEC methods that are optimally adapted to the properties of the transmission channel, so that an efficient utilization of the only slight redundancy is achieved.
The employment of some standard FEC methods in 64 QAM radio link systems is described in the following references:
M. Kavehrad, "Convolutional Coding for High-Speed Microwave Radio Communications", AT&T Technical Journal, Vol. 64, No. 7, Sept. 1985, pp. 1625-1637; S. Bellini et al., "Coding for Error Correction in High Capacity Digital Radio: An Application to 64 QAM Systems", European Conference on Radio-Relay Systems (ECRR), Munich, Nov. 1986, pp 166-172 and T. Noguchi et al., "6 GHz 135 MBPS Digital Radio System with 64 QAM Modulation", International Conference on Communications (ICC), Boston, June 19-22, 1983, Vol. 3, pp 1472-1477.
A convolutional self-orthogonal code (CSOC), a Reed-Solomon Code or, respectively, a block code having Lee metrics are used for error correction in these examples. Significant disadvantages of these methods are that they use a relatively high redundancy, utilize redundancy only inadequately for error correction and/or require a considerable system outlay.
The reference by G. Ungerboeck, "Trellis-Coded Modulation with Redundant Signal Sets", Parts I and II, IEEE Communications Magazine, Vol. 25, No. 2, Feb. 1987, pages 5-21 discloses an efficient FEC method wherein the redundancy is absorbed by a doubling of the number of steps and wherein no increase of the transmission rate is required. This method, however, requires the employment of an involved Viterbi decoder. Given a high number of levels, moreover, a further doubling thereof makes it susceptible to unavoidable system imperfections such as, for example, a non-ideal carrier recovery.
A redundancy-saving FEC method that is employable for QAM systems with Gray coding is presented in the reference by P. Mecklenburg et al, "Correction of Errors in Multilevel Gray-Coded Data", IEEE Trans. Inf. Theory, Vol. IT-19, No. 3, May 1973, 336-340. The n bits of each and every symbol to be transmitted are first compressed to i&lt;n bits by exclusive-OR operations. Finally, only these i bits are involved in a so-called underlaid coding as representatives of the transmission symbols. For radio link systems having a high number of levels, the method has the advantage that the overall system redundancy is noticeably lower due to the compression than is the effective redundancy of the underlaid code, namely all the more noticeably the higher the number of levels. What is disadvantageous is that the method is dependent on a Gray coding of the symbol points in the modulation plane. It can therefore not be used in systems that, for example, require a differential coding for the elimination of quadrant ambiguities or that do not employ a Gray-codeable (for example, hexagonal) modulation strategy.